1. Field of the Invention
This invention relates to proportional control systems, and more particularly to a proportional plus integral controller which avoids reset windup.
2. Description of the Prior Art
Proportional controllers are employed in process control systems to enable a controlled system variable to be maintained at a system set point by providing a feedback signal related in amplitude and polarity to deviations of the controlled variable from the set point.
In such systems, the controlled variable is monitored by a condition sensing device which provides a linearly changing command signal indicative of deviations of the controlled variable from the system set point. A process control apparatus is responsive to the command signal to effect an adjustment in the value of the manipulated variable to maintain the controlled variable at the system set point.
In known proportional controllers which provide only proportional action, the deviation of the controlled variable from the set point, or error, is never fully eliminated since it is necessary for an error to exist to command a system output other than the one neutral output at which the system was calibrated. This inherant deviation between the actual process condition, or controlled variable, and the desired process condition, or set point, is commonly referred to as "droop". In comfort control loops, for example, a relatively small droop can be felt by human occupants of a temperature controlled space.
Droop may be lowered through the use of a high gain amplifier in the proportional control loop. However, system stability requirements place a limit on the highest attainable gain and thus, on the minimum droop. Droop is eliminated through the use of controllers which provide proportional plus automatic reset action, sometimes called integral action.
Although proportional plus integral controllers provide the desired elimination of droop, control loops empolying an automatic reset control function are characterized by large overshoots in the event of process upsets as may be caused by start-up or large changes in load or set point, for example.
In some applications, it is desirable to prevent a proportional plus integral controller from acquiring a saturated condition in the integrator if the system being controlled is in a saturated condition such that the controlled variable is outside the control band for an extended period. For example, in the case where a proportional plus integral controller is employed to control the operation of a steam boiler or chiller a large overshoot may occur when the system is overloaded. Such condition occurs, for example, when a steam boiler operating at maximum capacity under heavy load cannot hold the steam pressure at the system set point so that the pressure is several degrees below the normal control set point.
If then the load is suddenly dropped, the pressure returns to the set point rapidly, but an integrator in the system, having accumulated maximum output, cannot begin to unwind until the set point is passed and the steam pressure is caused to increase above the set point. During the unwinding of the integrator, the system continues to operate at high capacity and the excessive overshoot may cause damage.
Therefore, it is desirable to have a proportional plus integral controller for use in a proportional control system, which provides an anti-reset windup function to eliminate the effect of reset windup in an integrator of the proportional plus integral controller.
It is also desirable to provide a proportional plus integral controller which may be included in an existing proportional control loop by simply cutting the lines between a controller sensor and an actuator controller.
It is also desirable to be able to retain the set point and proportional gain control of such existing proportional control loop in the proportional sensor controller.